The present invention relates to a method for surface structuring of a component through an ion beam, to a method for treating an optical element, and to optical elements treated with an ion beam, including mirrors, for optical systems in microlithography applications.
In the state of the art different methods for treating materials and components with ion beams are known.
Thus, it is known, e.g. to use focused ion beams (FIB) for imaging and manipulating surfaces. For these methods, acceleration voltages for ions, like e.g. gallium in the range of 5 to 50 kV, and corresponding currents of 2 pA to 20 nA are being used. The ion beam can be focused with electrostatic lenses to a diameter of a few nanometers (nm), and can then be guided over the surface, line by line, through respective deflection.
Through the interaction of the ion beam with the surface, so-called sputter processes (atomization processes) occur, which lead to the ability to treat materials in the nm range.
However, this method cannot be used for topography corrections of optical elements, due to the direct removal of the surface, since, due to a local use of this method, also the micro-roughness is locally changed.
Furthermore, it is known e.g. to use ion beam methods with lower acceleration energies, this means ions with energies in the range of 0.2 keV to 1.2 keV for treating surfaces of optical elements, like e.g. lenses for objectives in microlithography applications. Herein, a lower acceleration voltage is used, compared to the focused ion beam method, so that only a lower removal occurs directly in a layer of 1 to 2 nm from the surface. Thereby, it can be accomplished, that the micro-roughness of the surface is maintained, and only larger size topography errors are corrected. However, this method has lower efficiency, due to the lower removal rate. Furthermore, in the correction of topographic errors, in the range of <1 mm, there are problems with positioning precision, since ions in this energy range are hard to focus.
Furthermore, also high energy ion beam methods are known, in which ions are implanted in components or materials with acceleration energies of up to 3 MeV, or more. This method of ion implantation is mostly used for doting semiconductors.
From DE 41 36 511 C2 a method for producing a Si/FeSi2-heterostructure is known, wherein iron ions are implanted into a silicon substrate with the iron ions being irradiated with an energy of 20 keV to 20 MeV onto the substrate.
DE 38 41 352 A1 discloses e.g. the implantation of boron, carbon, nitrogen, silicon or hydrogen ions in a silicon-carbide layer during the production of a silicon carbide diaphragm for a radiation lithography mask. Herein ion implantations serve the purpose to achieve a stress relaxation and better optical transparency in an oxide layer formed during subsequent temperature treatment.
The U.S. Pat. No. 4,840,816 describes doping of crystalline oxides like LiNbO3 with heavy metals for forming a beam waveguide. The ions are implanted with a doping density in the range of 1.2×1017 to 2.5×1017 ions per cm2 with energies of about 360 keV at a temperature of −190° C.
Due to these different areas of application, the basics of the interaction of ion beams with materials have already been researched intensively. From this research it is known that the ions are slowed down, when impacting the material, through various braking mechanisms, like inelastic collisions with bound electrons, inelastic collisions with atom nuclei, elastic collisions with bound electrons, and elastic collisions with atom nuclei etc. An overview of the resulting macroscopic and microscopic effects in amorphous silicon dioxide is given e.g. in the publication by R. A. B. Devine in “Nuclear Instruments and Methods in Physics Research” B91 (1994) pages 378 to 390.